I also like the idea of using BTC you mentioned in another post to state good intentions, but acquiring BTC to use in the first place seems like much more hassle, registrations and sharing of data is required.
You can get btc with cash, if you already have it,it's a nice option.
"If it can be abused,it will be abused".
Captchas and small btc payments can't easily be abused against the user but email and phone number can. Email and phone number are also not costly to an attacker to spoof or churn out at a high rate. Captchas and small payments have accumulating prohibitive cost for an attacker.
IfI create a gmail just to sign up,it certainly is much more of a hassle. Even temporary inbox services are a hassle these days compared to a 20sec captcha.
That Email is a hassle but not a big one is exactly the point why sites ask for one.
And Captcha? You talked about spying before and this is exactly what captcha does these days. Its massively invasive, plus it was always annoying and never really worked well for its intended purpose.
I have to say, it seems a bit like your argument is stuck a few decades ago. While I quite like the thought experiment of how the internet could work with essentially anonymous and frictionless participation, I don't think it ever has or at least not for decades. And examples like hn and reddit are interesting, but every other blogs open comments section show that this working is rather the exception than the norm.
I consider myself privacy conscious, and from the options captcha, email and BTC, email for me strikes the right balance of ease of use, anonymity and effectiveness. More options would always be welcome though. Conecpts like Indieauth for example, look promising, but let's not kid ourselves, privacy wise, they require more of me than providing a fake address to get an email. And just like the BTC solution, it would probably keep many more people out if it were the only option.
But I also think asking for at least an email is not that unreasonable. Sure, I value my privacy, but I also value a good community and keeping the worst spammers out often helps. Talking smaller instances here, obviously, but I think that's where the value is with something like mastodon. Tight knit communities with the option to reach out to others.